1. Field of the Invention
The present invention relates to a control apparatus which is well suited for application in case of executing a skip downshift in an automatic transmission of so-called "twin clutch type".
2. Description of the Prior Art
A power-ON skip downshift is usually executed when the driver of a motor vehicle has pressed the accelerator pedal thereof rapidly and deeply. On the other hand, a power-OFF skip downshift is usually executed when the driver manipulates the shift lever of the motor vehicle in want of engine braking.
The official gazette of Japanese Patent Application Laid-open No. 121362/1991 discloses a control technique wherein, in executing the power-ON skip downshift of, for example, the fourth speed stage (higher speed stage).fwdarw.second speed stage (lower speed stage) of an automatic transmission, the gearshift control is performed via the intermediate stage (third speed stage) of the automatic transmission, and a command for the intermediate stage is canceled before this intermediate stage is completely achieved.
In executing the skip downshift, when the intermediate stage is involved to successively perform the first gearshift from the higher speed stage to the intermediate stage and the second gearshift from the intermediate stage to the lower speed stage, the r.p.m. or rotational speed of each rotary member can be smoothly changed correspondingly, and a gearshift shock can be alleviated.
Moreover, on that occasion, the whole gearshifting period of time required till the completion of the skip downshift can be shortened by applying the technique as disclosed in the above official gazette of Japanese Patent Application Laid-open No. 121362/1991.
Meanwhile, there has heretofore been well known an automatic transmission of so-called "twin clutch type" including two clutches which are arranged in parallel with a transmission input shaft, and a plurality of gear trains which are arranged on the respective output shafts of the two clutches and which are selectively connected with a transmission output shaft by synchro mechanism. With the automatic transmission of this type, a predetermined gearshift is executed by the combination of the selective connection of the gear trains and the clutch-to-clutch changeover of the two clutches.
In order to attain the alleviation of the gearshift shock and the shortening of the gearshifting time period, the automatic transmission of such twin clutch type is usually so controlled that the gear train corresponding to the next gearshift stage anticipated is connected on standby beforehand, whereupon the actual gearshift can be implemented by only the changeover between the two clutches. The changeover between the two clutches is the so-called "clutch-to-clutch changeover".
The control technique concerning the power-ON skip downshift as disclosed in the above official gazette of Japanese Patent Application Laid-open No. 121362/1991 is so constructed that a one-way clutch is used for achieving the intermediate stage. Accordingly, the prior-art technique has the problem that it cannot be simply applied to an automatic transmission of the type in which the intermediate stage is also achieved by the clutch-to-clutch operation. Moreover, in a case where the automatic transmission is one of the so-called twin clutch type, circumstances are further complicated. Therefore, the prior-art technique has the problem that it cannot be applied at all.
More specifically, the twin clutch type automatic transmission is under the circumstances that the first gearshift from the higher speed stage to the intermediate stage is the clutch-to-clutch gearshift from one of the clutches to the other, while the second gearshift from the intermediate stage to the lower speed stage is the clutch-to-clutch gearshift from the other clutch to the "original" clutch (one clutch). Further, it is under the circumstances that the synchro mechanism needs to be switched over midway of the above operation of the clutch-to-clutch gearshifts. Therefore, the technique as disclosed in the official gazette of Japanese Patent Application Laid-open No. 121362/1991 has heretofore been hardly applicable to the automatic transmission of the twin clutch type.
It might be suggested to control the automatic transmission of this type so that, when a request for the skip downshift has been made, the entire gearshift may be executed by only the switchover of the synchro mechanism without. the aid of the intermediate stage, in other words, without the operations of the two clutches. Such a control, however, cannot be adopted for the reason that the switchover of the synchro mechanism is actually impossible.
Accordingly, the automatic transmission of this type has heretofore been controlled in compliance with the request for the skip downshift by performing merely in succession the three operations, that is, implementing the first gearshift of the higher speed stage.fwdarw.the intermediate stage, thereafter switching over the synchro mechanism, and subsequently implementing the second gearshift of the intermediate stage.fwdarw.the lower speed stage.
The control method of the mere succession of the first gearshift.about.the switchover of the synchro mechanism.about.the second gearshift, however, has the problem that the time period required till the end of the entire gearshift becomes very long notwithstanding the skip downshift is usually executed when the gearshift is to be completed as soon as possible.